The city’s tallest building by 2020 will be the same as it is today: 200 Clarendon (the former Hancock), which stretches to 790 feet at its tallest architectural detail, not counting antennae and other apparatus.

The second-tallest will also be the same, too: The 749-foot Pru.

The third will likely be new: The under-construction One Dalton hotel-residential tower in Back Bay is slated to stretch to 742 feet at its highest architectural feature.

What’s holding the city back from building to 800 feet, 900 feet, (gasp!) 1,000 feet? The sagas of two arrested developments—both garage conversions, in fact—illustrate the answer.

Developer Millennium Partners had at one point proposed building a 775-foot mixed-use tower, including condos, at the site of the shuttered Winthrop Square Garage at 240 Devonshire Street. (Millennium bought the garage from the city, and originally proposed a still-significant 750-foot project, before bumping that to 775 feet in November 2016.)

That was until February 2018, when the Boston Planning and Development Agency proposed allowing Chiofaro to build a tower as high as 600 feet in exchange for establishing a fund worth up to $30 million to compensate the aquarium for any disruptions and to set aside parking for the institution.

All in all, the plan would allow for the tallest tower ever along Boston Harbor. Chiofaro appears amenable to the deal’s terms, including the aquarium’s indemnification against loss of visitors/business.

Still, the tower might yet tarry: The state could take up to two months to review the BPDA plan; then it’s another round of local and state permitting; and Chiofaro hasn’t settled on a final design to even propose.

There is also continuing opposition from the owners in the neighboring Harbor Towers complex.